Grainy Slo-Mo

Today I was filming a show using the 120fps slo-mo option, and noticed at the end that the video footage was rather grainy. I'm not sure if this was just because of the low-light situation or an issue with my phone itself. Please check the image below and let me know your opinions. I'm leaning towards low-light issues, however i'm just touching base here for some confirmation to put my mind at ease.